If you're going to get ahead, you have to know yourself inside out. If you don't, how will anyone else? And more importantly, how will you improve?
This can be related to anything, but with writing it's absolutely crucial that you know all your own strengths and weaknesses. Where do you excel, and where do the notes you're getting back keep finding fault?
I'm going to bare what I think are my own ones: overwriting description, not having terse enough action. My dialogue can be funny but it's not punchy, laugh-out-loud stuff. And Act 2 can be a very dodgy place.
And the good stuff: my scripts have good structure. I'm good at coming up with ideas and at thinking of ways to fix things in rewrites. Actually, I quite enjoy doing rewrites, in a perverse way.
So how do you fix your writing sins once you identify them? Three ways. Having problems writing comedy dialogue? Watch some very funny films, and then read the scripts for good measure. Overwriting description is harder to fix; you just have to go over your script and keep asking yourself if every line is necessary. For overall troubleshooting, get your script read, either by a pro reader or by a fellow writer. They'll notice things you've missed, and they'll be more objective than you can hope to be.
Remember, a bad writing habit identified is one that's one step closer to being fixed...